1. Field of the Invention
The present invention relates to a shadow mask in a color cathode ray tube, and more particularly, to a shadow mask formed with an invar alloy thin plate having a texture that allows for formation of uniform sized electron beam pass-through holes, with excellent roundness and a small etching deviation in etching.
2. Discussion of the Related Art
Referring to FIG. 1, a color cathode ray tube (CRT) is provided with a panel 1 having a fluorescent film 3 coated on an inside surface thereof, a funnel 2 having conductive graphite coated on an inside surface thereof and fusion welded to the panel 1 with fusion glass at a temperature of approx. 450.degree. C., an electron gun 6 mounted to a neck portion 4 of the funnel 2 for emitting electron beams 5, a shadow mask 7 being a color selecting electrode supported by a frame 8 inside of the panel 1, and deflection yokes 9 mounted on an outer circumference of the funnel 2 for deflection of the electron beams. The numeral 10 denotes an inner shield.
When a video signal is provided to the aforementioned color cathode ray tube, thermal electrons 5 are emitted from cathodes in the electron gun 6 and travel toward the panel 1, while being accelerated and focused by different electrodes in the electron gun 6. In the travel, the electron beams 5 are deflected, causing changes in their travel path, by a magnetic field generated by the deflection yokes 9 on the neck portion 4 of the funnel 2, thus scanning an entire surface of the panel 1. The deflected electron beams 5 are used to represent a color as they pass through a slot in the shadow mask 7 supported from an inside frame of the panel 1 since those electron beams collide with different fluorescent films 3 on the inside surface of the panel 1, to generate light, thereby reproducing the video signal.
A rimmed steel in JIS G3141 series or an aluminum killed steel(AK steel), each being a pure iron, has been used as a material for fabricating shadow masks 7 in a color cathode ray tube. However, due to the large thermal expansion coefficients of these materials(pure steel: 11.5.times.10.sup.-6 deg.sup.-1) and because of current developments in high definition television TV technology, thermal expansion of the shadow mask 7 resulting from heat caused by collisions between the electrons emitted from the electron gun and the shadow mask causes doming, which is a color dispersion experienced when electron beams collide with a fluorescent surface corresponding to a color other than a designated color due to the thermal expansion. In order to prevent doming, an invar alloy in Fe--Ni series is used, which has a smaller thermal expansion coefficient(1.5.times.10.sup.-6 deg.sup.-1).
The shadow mask 7 is formed as follows.
A slab, formed from casting of a molten steel having an invar composition in a converter or an electric furnace, is subjected to hot rolling, annealing, acid cleaning and cold rolling, thereby forming a thin plate with a thickness of 0.1.about.0.5mm. In the cold rolling, a rolling process may be performed several times to achieve a desired reduction ratio. Then, an intermediate annealing process is conducted at a temperature over 800.degree. C., where the slab is temper rolled to control the thickness and surface roughness, then annealed. The surface is cleaned and dried, a coat of photoresist is applied, exposed and developed, etched by a ferrous chloride solution, removed, cut, etc. to obtain a plate with holes. The plate is then cleaned, dried, annealed at a temperature over 800.degree. C., hot pressed, black iron oxide coated, weld assembled and packed, to obtain a shadow mask as shown in FIG. 1.
As the shadow mask of invar alloy has a small thermal expansion coefficient, facilitating an exact pass of the electron beams irrespective of a temperature, the invar alloy is widely used as a material of shadow masks suitable for displays of high definition TV broadcasting systems and computers which require a high definition still image. In order to obtain a high definition shadow mask of such an invar alloy, small pitched uniform holes should be formed in a shadow mask material by etching. However, despite its low thermal expansion coefficient, invar alloys are known as materials which are not etched well and which therefor present problems in obtaining uniform holes. For these reasons, the etching of invar alloy has been an important subject to be solved.
For example, Japanese laid open patent No. S61-82453 restricts the carbon content to below 0.01% and Japanese laid open patent No. S61-84356 restricts the non-metallic contents, each attempting to improve the etching property. And, Japanese patent publication No. S59-32859, Korean patent publication No. 88-102 and 87-147 and U.S. Pat. No. 4,528,246 each disclose that a shadow mask material fabricated using an invar alloy with over 35% of {100} texture, which is obtained by controlling the cold rolling and annealing in a shadow mask raw material forming process, permits good etching characteristics to facilitate formation of uniform electron beam pass-through holes, resulting in a reduction in the doming and thus improved color reproduction. However, the background invar alloy material used in these related art systems shows S, B, N impurities, even when the carbon content is below 0.01%. Since the impurities are segregated from crystal grains or exist as interstitial atoms in crystal when annealed, affecting etching, it is important to control these impurities.
A {100} crystal plane has the fastest etch rate. Therefore, if the {100} planes are concentrated on a rolled surface, the etching can be carried out efficiently. However, if the {100} crystal plane concentration is very high, the fast etching causes formation of non-round holes, particularly, if the concentration is over 90%, and the holes are formed by etching along the crystal lattice, resulting in formation of holes which are neither round nor uniform. Therefore, the 35% concentration of the {100} crystal planes disclosed by these references may not be a satisfactory crystal orientation for etching.
Japanese patent publication No. S62-229738, S62-103943, U.S. Pat. No. 4,771,213 and Korean patent publication No. 90-9076 disclose a shadow mask designed to have crystal planes with a greater a-value (a-value=I{100}I/{110}) to face the screen, where I{100} is a diffraction intensity at a {100} crystal plane and I{110} is a diffraction intensity, at a {110} crystal plane, and the g-value should be greater than 2 where g-value=I{100}+I{111} in comparison to I{110} when the I{111} is a diffraction intensity at a {111} crystal plane, i.e., g=(I{100}+)I{111}/I{110} should be greater than 2. The aforementioned patents are not necessarily teaching the correct overall distributions of the crystal planes because integrated intensities used for calculating a-, and g-values and (hkl) integrated intensities are not arithmetically related to the frequency of crystal planes, and are assessed only on a particular place of the shadow mask. That is, for example, even if crystal planes of {200}, {111}), {220} and {311} are present each by 25% on a surface of the shadow mask of invar alloy, the values of g and a may not be 2 and 1, respectively. This is because the diffraction intensity varies with a structural factor F which is indicative of the arrangement of electrons and atoms in a substance, the multiplicity factor P, temperature and absorption factors. These are generally expressed by the equation shown below. ##EQU1##
Where, I=a relative integrated intensity(arbitrary units), F=a structural factor, P=a multiplicity factor, and .theta.=a Bragg angle.
According to this equation, even with a random orientation of crystal grains, the diffraction intensity at each diffraction plane is not uniform as shown in TABLE 1 below(JCPDS CARD NO. 23-297).
TABLE 1 Ratio of diffraction intensity of random distributed sample (JCPDS CARD NO. 23-297) (hkl) 111 200 220 311 222 400 intensity ratio I/Io 100% 80% 50% 80% 50% 30%
Even in the case of random orientation of crystal grains, g=3.6 and a=1.6 are obtained when they are calculated according to Japanese laid open patent No. S62-229738, U.S. Pat. No. 4,771,213, Korean patent publication No. 90-9076 and Japanese laid open patent No. S62-103943. This implies that there should be more {220} crystal planes on the surface of the plate in question for g=2.about.3.6, i.e., there should be less {200} or {111} crystal planes. This leads to a conclusion contrary to that taught by the patents that there is an abundance of texture having many {200} crystal planes concentrated on the surface of the plate if g is greater than 2. In order to rectify this matter, the measured integrated intensity should be corrected (for example, correct to an integrated intensity of a powder sample having a random orientation of crystal grains) before use. And, though the patents teach an integrated intensity of an X-ray diffraction pattern of a crystal, the integrated intensity may not precisely represent the intensity at a crystal plane. Measurements of diffraction intensities as well as accurate analyses of crystal grain distributions for crystal planes {111}, {200}, {220} and {311} are required.